ENH: Data availability check

[KCollins]

This might be just a function to see if any data is available for a given magnetometer and timeframe (I wonder if the SuperMAG API already has such a thing?) - May also extend to a Gantt-style availability chart, similar to what we use for the Grape. I'd like to be able to include ground mag availability as a parameter in the THEMIS HSJ dataframe, e.g., so that I can filter out those events for which we don't have data.